The present invention relates generally to inkjet apparatus, including inkjet printing mechanisms, and more particularly to improved mechanism and method for avoiding print head crashes in such apparatus.
Inkjet printing mechanisms may be used in a variety of different inkjet apparatus, such as plotters, facsimile machines, copiers, and inkjet printers collectively referred to in the following as printers, to print images using a colorant, referred to generally herein as xe2x80x9cinkxe2x80x9d. These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpensxe2x80x9d or xe2x80x9cprint headsxe2x80x9d to shoot drops of ink onto print media, which can be used in the form of cut sheets or rolls of print media, which may include paper, vinyl, films, canvas or the like, in a variety of different dimensions.
Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the print head carriage across the print zone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of print head travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the print head cartridge. In multi-color cartridges, several print heads and reservoirs are combined into a single unit, with each reservoir/print head combination for a given color also being referred to herein as a xe2x80x9cpen.xe2x80x9d
Each pen has a nozzle plate that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the print head may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal print head technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
By selectively energizing the resistors as the print head moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located generally side-by-side on the print head, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the print head made one complete traverse through the print zone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink that can be laid down in a single pass.
For placing further print swaths on the print media, a print media feed mechanism is employed to advance or index the medium in the print zone in a second direction, called the media direction, which is usually substantially perpendicular to scanning direction of the print head.
Thus, to print an image, the print head is scanned back and forth across a print zone at a very close distance above the sheet, with the pen shooting drops of ink as it moves. On one hand, for instance, the distance between the printhead and the paper must be as small as possible, for example less than 1.7 mm, in order to obtain an accurate positioning of the ink dots projected from the printhead and to avoid spraying artefacts.
However, when a lot of ink is placed on some print media (especially on low cost paper based media) the print media may be subject to a phenomenon known as xe2x80x9ccocklexe2x80x9d. In existing printers, cockle results from the print media swelling and expanding as it absorbs water contained in the ink, whilst the print media is simultaneously constrained against lateral expansion due to being gripped at given locations along the scan axis (i.e. along the axis of movement of the print head), between the pinch wheels and the main drive roller.
This results in the formation of undulations or wrinkles in the plane of the print media. As a consequence, the distance between the print media and the print head decreases at some localized points. This phenomenon is especially noticeable when printing area fills of more than 200%. By this it is meant that in a given area of print media, the amount of ink deposited during the printing operation is two or more times the quantity of ink that is required to cover that area.
If the degree of cockle is particularly severe, a xe2x80x9cbubblexe2x80x9d in the media may form. If the height of the media bubble is sufficient, the plot may be damaged as ink on the plot is smeared by the print head. Indeed, in more severe cases, a media crash may occur as the print head impacts against the print media itself. A media crash may seriously affect the subsequent print quality or throughput of the printer due to damaging the operation of individual nozzles of the pen. In some cases a media crash may necessitate the replacement of the pen.
This problem is of particular concern where a printer prints a series of plots from a roll of print media, when unsupervised by a human operator, as is often the case in commercial printing operations. This is because the print media expands in a cumulative manner. As a consequence, the chance of a media bubble being generated increases with each successive plot until the pinch rollers are released allowing the media to xe2x80x9crelaxxe2x80x9d and so to flatten once again against the platen of the printer. Thus, even if a printer may print a single high density plot without risk of a media bubble forming, it may be at risk of a print media crash if it is left unattended to print a series of plots form a role of print media. As the skilled reader will appreciate, if the media bubble should grow sufficiently, a media crash may then arise.
Furthermore, even if the occurrence of a media bubble does not result in a print media crash, a further problem may arise. Once formed, during a printing operation a media bubble will generally expand in the opposite direction to that in which the print media is fed at approximately the rate at which the print media advances. Generally, this will continue for as long as the printer continues to print on a continuous sheet or role of print media. In such a situation, all plots printed on the same sheet or roll of print media, subsequent to the one in which the media bubble developed, will additionally be damaged. Thus, the amount of damage that may be caused, in terms of lost output and wasted supplies, may be considerable if a printer which is printing multiple plots from a roll is left unattended.
In view of the foregoing, it is an object of the present invention to provide an improved inkjet apparatus and an improved method for operating an inkjet apparatus.
A further object of the invention is to provide an inkjet apparatus and method for operating an inkjet apparatus for reducing the likelihood of a print head crashes, particularly when printing on roll fed print media and where the inkjet apparatus is not supervised by a human operator.
Still another object of the present invention is to provide an inkjet apparatus and method for operating an inkjet apparatus for reducing the damage to plots caused by the occurrence of media bubbles, particularly when using roll fed print media and where the inkjet apparatus is not supervised by a human operator.
To achieve these objects, the present invention provides for an inkjet apparatus and method for allowing print media, which has cockled, to relax and so to lie flat on the platen of the printer, without requiring the pinch rollers to be released from engagement with the main drive roller of the printer. According to the present invention there is provided a method of flattening print media in an ink jet apparatus, said apparatus comprising a media feed path, a media drive roller and one or more pinch rollers, said one or more pinch rollers being arranged to rotatably cooperate with said drive roller so as to grip said media therebetween, said drive roller having a rotational axis extending substantially transverse to said feed path and being arranged to rotate in first and second directions to respectively feed said media in first and second feed directions; said method comprising the steps of: feeding said media a first distance in said first feed direction; and then, feeding said media a second distance in said second feed direction, thereby causing said media to flatten in a direction substantially transverse to said feed path.
By implementing the media flattening method of the present invention in this manner several advantages are realized. Firstly, by periodically implementing the method of the invention, the print media will be left flat, without media bubbles. This means further ink may be deposited on the print media, without the increasing the risk of a media bubble forming, as has been described above. Because the method of the present invention may be implemented without the need to raise the pinch wheels, it may be implemented in an automated manner, without the need for an operator to be present. Consequently, a printer may be left unattended to print a series of images on a roll of print media, without the risk of a series of plots being damaged by the formation of a single media bubble. This feature of the present invention makes it particularly suitable for applications in which it is especially important to avoid wasting printer supplies and where unsupervised operation is desirable; for example commercial print operations.
Using the process of the present invention, the media may be left ready to be printed on again at the end of the process at the exact point where the previous print finished. Thus, it may be ensured that no print media is wasted, which is if course an important consideration in a commercial print operation.
The use of the present invention also allows printer operators to conserve costs by using cheaper paper, that is more normally susceptible to cockling and media bubbles in unsupervised printing operations.